IN 98 YEARS the National Zoo has come a far piece from the days when Smithsonian taxidermist William Temple Hornaday kept a menagerie behind the Castle on the Mall. Today about three thousand animals live on 163 acres in Rock Creek Park, and are viewed by three million visitors a year. There's a commissary, a hospital and, in Front Royal, a conservation-research center for endangered species.

There is room for yet more growth, even without expanding the park boundaries, says Michael Robinson, the zoo's 58-year-old director. Although Washington's zoo is among the 10 best in the country, Robinson says, it ranks as good rather than excellent; he wants to make it the best.

Robinson, himself a bit birdlike with a crest of fluffy blonde hair above a ruddy, genial face, has spent most of his professional life studying wildlife in the tropics. He became director in 1984 and retains a boyish enthusiasm for animals; two of the greatest pleasures of his job, he says, are feeding fish to the sea lions and standing next to Nancy, the African elephant.

If Robinson can maneuver his dreams into reality, zoogoers in the year 2000 will turn in off Connecticut Avenue to see groups of animals roaming an African plain that will "really look like the National Park outside Nairobi." Across the way will be an Indonesian rain forest; down in Beaver Valley will be an aquatic wonderland of sea otters, penguins and South American fish. Plants will be as much in evidence as animals, and relevant works of art and fossils will be interspersed with the fauna and flora.

Even now the zoo is moving in this holistic, naturalistic direction. More animals live in environments that approximate natural habitats, the worldwide trend in zoo management. On Monkey Island, for example, primates play on a flower-studded hillside; part of the island is surrounded by a moat where water lilies bloom and otters swim. Elsewhere there's a beaver dam across a stream. And construction is underway for a wetland in front of the Bird House and a new gibbon display that will allow the arboreal animals plenty of space to brachiate -- or swing from handhold to handhold. By the end of the decade, Robinson hopes, there'll be an Amazonian rain forest and river where the polar bears were. (The cold-weather bears have gone to zoos in Detroit and Chicago.) There's talk of issuing umbrellas at the entrance to the rain forest.

Another of Robinson's state-of-the-art concepts has already been translated into glass and concrete. The new Invertebrate House, which opened in May, combines dramatic displays of the spineless creatures of the world with large-print signs explaining them, magnifying glasses and microscopes, and a computer game to wrap up the experience. Here and there, art is juxtaposed to nature -- a reproduction of a Minoan vase with an octopus motif, for example, next to a tank containing the real thing.

Many innovations in zoo design, says Robinson and other zoo planners, may benefit visitors more than the animals. Even when it comes to imitating natural habitats, Robinson cautions, "We shouldn't assume that the same things that are important to us are important to them. We don't know that monkeys sit and think about bark on trees or leaves on trees or anything like that. Trees are places they use as bridges from one place to another, places they look for food. It's possible they're just as 'happy' with white plastic tubes as they would be with a real branch."

But one endeavor at the zoo these days is good for both people and animals. Zoo creatures, which don't need to hunt for food or protect themselves from predators, tend to become lazy. It's not healthy for them, and it disappoints zoogoers, who want to see animals moving about, being animals. So, keepers at the National Zoo look for ways to perk up their charges, to make their lives a little more difficult and a lot more interesting. Recently, the coatis, long-nosed, raccoon-like animals, underwent a radical change in lifestyle. Previously their meals had been served on the proverbial silver platter; the coatis were inactive 70 percent of the day. Now keepers bury crickets under wood-chip mulch; the small mammals are kept busy hunting and digging.